1. Field of the Invention
This invention relates to a fuel injection system for a diesel engine, and more particularly to a fuel injection system for a diesel engine having a pintle fuel injection nozzle which has a valve needle lifted by the fuel pressure and in which the effective area of the spray hole or the spray area is changed according to the lift of the valve needle. In this specification, the term "pintle fuel injection nozzle" should be broadly interpreted to include throttle type nozzles in which the so-called "throttle range" in which the valve needle throttles the spray hole extends over a relatively large part of the lift of the valve needle.
2. Description of the Prior Art
There have been proposed various fuel injection nozzles in which the lift of the valve needle is suppressed to throttle the spray hole during light load operation of the engine, thereby promoting atomization of sprayed fuel at light load. This type will be referred to as the "needle-lift-suppressing type", hereinbelow. As one kind of needle-lift-suppressing type fuel injection nozzles, there has been known a central plunger type pintle fuel injection nozzle as disclosed in Japanese Unexamined Patent Publication No. 57(1982)-151058. In the central plunger type pintle fuel injection nozzle disclosed in this Patent Publication, a plunger member is provided behind a valve needle to be slidable coaxially with the valve needle, and by imparting a predetermined pressure to the plunger member, lift of the valve needle is suppressed after a predetermined lift of the valve needle (pre-lift) so that the lift of the valve needle is maintained in the throttle range for a predetermined time interval, thereby promoting atomization of sprayed fuel and changing the fuel injection rate (the amount of fuel injected per unit crank angle). In the fuel injection nozzle of this type, the amount of the pre-lift is not completely fixed but is self-controlled according to the fuel pressure so that the amount of the lift of the valve needle is increased with increase in the fuel pressure as shown in FIG. 11. The self-control arrangement of the valve needle lift is advantageous in that fluctuation in the effective area of the spray hole from cylinder to cylinder due to deposition of carbon or variation in the machined dimensions of the spray holes, for instance, can be compensated for.
However, the conventional central plunger type pintle fuel injection nozzle has a drawback in that since the amount of the pre-lift solely depends upon the fuel pressure fed from the fuel injection pump, it can be matched to only a particular operating condition of the engine, and cannot properly use functions for atomization of fuel and/or for changing the fuel injection amount according to various operating conditions of the engine. Thus, in the conventional central plunger type pintle fuel injection nozzle, it is difficult to control the lift of the valve needle in order to obtain improved combustion in the combustion chamber, to improve the engine output power, and to obtain improved emission control according to the operating condition over a wide operating range of the engine.
For example, the amount of fuel injected from the fuel injection pump is generally increased upon cranking (starting-supercharging) as indicated at A in FIG. 9 in order to ensure good starting performance. In a fuel injection nozzle which is not of the needle-lift-suppressing type, the amount of fuel injected from the nozzle is actually increased corresponding to discharge characteristics of the fuel injection pump as shown by the solid line in FIG. 10, whereby the cranking time is shortened. On the other hand, in the conventional central plunger type pintle fuel injection nozzle or in other needle-lift-suppressing type fuel injection nozzles, the amount of fuel injected from the nozzle is not so increased as in a nozzle not of the needle-lift-suppressing type even when the amount of fuel injected from the fuel injection pump is increased according to the characteristics shown in FIG. 9, since the needle lift is suppressed at low load and low speed as upon cranking. (See the dotted line in FIG. 10.) Accordingly, the cranking time required before complete combustion starts is not sufficiently shortened. This is especially significant when the ambient temperature is low, and heavy load is exerted on the battery.
The following table shows a comparison in the starting abilities between the central plunger type pintle fuel injection nozzle (one of the needle-lift-suppressing type nozzles) and nozzle which is not of the needle-lift-suppressing type).
______________________________________ time required to time required to start start firing complete combustion ______________________________________ central plunger type 1.5 to 1.8 sec 30 to 35 sec pintle nozzle nozzle not of the 2.0 sec 24 sec needle-lift suppressing type ______________________________________
The "time required to start firing" represents the time interval for which the engine has to be cranked before firing starts and the "time required to start complete combustion" represents the time interval for which the engine has to be cranked before complete combustion starts. As can be understood from the table, though the difference in the time required to start firing between the two types of nozzles is relatively small, the time required to start complete combustion is substantially longer with the central plunger type than with the nozzle not of the needle-lift-suppressing type.
As needle-lift-suppressing type fuel injection nozzles other than the central plunger type pintle fuel injection nozzle, there have been known one in which the needle lift is limited at low load and low speed by changing the position of the stopper, one in which the needle lift is limited at low load and low speed by imparting to the valve needle a hydraulic pressure other than the fuel pressure from the fuel injection pump, and the like. As will be apparent to those skilled in the art, the aforesaid problem arises also in needle-lift-suppressing type fuel injection nozzles other than the central plunger type pintle fuel injection nozzle.